This invention pertains to a method of controlling properties of a ferromagnetic samarium substance whereby the substance becomes suitably used as a spin-resolving device for charged particles and also pertains to a ferromagnetic material adapted to be controlled by this method and a spin-resolving device making use of the relevant properties.
Magnetic substances are extensively used in various fields as permanent magnets, soft magnetic materials, and magnetic recording media. When magnetic substances are used as such materials, the magnetization or a magnetic field generated thereby, the interaction with an external magnetic field, and so on, are essentially utilized.
On the other hand, coupled with the recent researches on the particle-spin dependence of various interactions in the fields of high-energy physics, solid-state physics, and so on, an improvement of the technology to produce the spin-polarized particles or to measure the spin polarization has been desired.
In a magnetic substance, the electron spin is spatially polarized in a temperature range below a specific temperature called the magnetic transition point. Therefore, it is naturally thought that magnetic substances could be suitable materials for a spin-resolving device for charged particles.
Practically, however, magnetic substances, especially ferromagnetic ones, are rarely used in the spin-resolving technology; but instead, for example, a semiconductor excited by a circularly polarized laser beam is used as a polarized electron-beam source, and the diffraction and/or scattering of an electron beam from heavy elements such as gold, tungsten, etc., or the emission of a circularly polarized light from a GaAs film irradiated with an electron beam is used in the measurement of the spin polarization of an electron beam (see JP08-248141).
The main reason for the inapplicability of ferromagnetic substances to the spin-resolving device resides in the fact that the ferromagnetic ordering of the electron spin over a macroscopic range is usually accompanied with the generation of a finite magnetization. The magnetization of a ferromagnetic substance is unfavorable for the application to the spin-resolving device in the following two points.
Firstly, under no magnetic field, a ferromagnetic substance usually tends to split into many small domains with different directions of the electron spin polarization to energetically stabilize. In other words, the macroscopic spin polarization does not spontaneously occur despite the ferromagnetic nature. And, it is difficult in general to control and evaluate such a magnetic-domain structure.
Secondly, when a ferromagnetic spin polarization is attained over a macroscopic range somehow, the ferromagnetic substance itself generates a stray magnetic field. In the case that an external magnetic field is applied to achieve the macroscopic spin polarization, there also exists a stray magnetic field from an apparatus generating the external field. Such stray fields might affect the charge and/or the spin magnetic moment of a spin-polarized charged particle in the form of the Lorentz force, the magnetic force, and the Larmor precession.
One object of the invention is to provide a method of controlling properties of a ferromagnetic material, whereby it becomes suitably available for the spin-resolving technology for charged particles, which has no undesirable effect on the charge and/or the spin magnetic moment of the charged particle.
Another object of the invention is to provide a ferromagnetic material, where the properties suitable for the spin-resolving technology are easily materialized and controlled.
Further object of the invention is to provide a spin-resolving device for charged particles, which comprises a ferromagnetic material and, nonetheless, is free from the formation of magnetic domains and the leakage of a magnetic field.
A first feature of the invention is to control properties of a ferromagnetic substance, whose magnetization arises mainly from the element of samarium and in a method of this feature, the present ferromagnetic substance is so controlled as to keep a ferromagnetic alignment of the electron spin over a macroscopic range and to have a spin-orbital compensation property characterized by little or no magnetization due to the compensation between the two parts of the magnetization originating from the orbital magnetic moment and the spin one.
This spin-orbital compensation property can be attained at an arbitrary temperature by an appropriate modification of the composition of the ferromagnetic samarium substance and an appropriate preliminary thermomagnetic process.
The composition of the ferromagnetic samarium substance is controlled by, according to the relation in size between the partial magnetization originating from the orbital magnetic moment and that originating from the spin one, replacing a part of samarium in the base substance with other elements or adding other elements to the base substance so as to reduce the total magnetization.
A second feature of the invention lies in a ferromagnetic material, whose magnetization arises mainly from samarium and adapted to keep a ferromagnetic alignment of the electron spin therein over a macroscopic range and have a spin-orbital compensation property characterized by little or no magnetization due to the compensation between the two parts of the magnetization originating from the orbital magnetic moment and the spin one.
This ferromagnetic material is based on a ferromagnetic substance, whose magnetization arises mainly from samarium, and the composition of the ferromagnetic material is controlled by, according to the relation in size between the partial magnetization originating from the orbital magnetic moment and that originating from the spin one, replacing a part of samarium in the base substance with other elements or adding other elements to the base substance so as to reduce the total magnetization.
A third feature of the invention is to generate a spin-polarized charged-particle beam, measure the degree of the spin polarization or polarize or analyze the spin for a charged-particle flow (an electric current), making use of a spin-orbital compensation property of the ferromagnetic material, which serves as a spin-resolving device for charged particles.
As aforementioned, the ferromagnetic material of the invention comprises a substance, whose magnetization arises mainly from samarium, (referred to as xe2x80x9cferromagnetic samarium substancexe2x80x9d hereafter) and the property suitable for the spin-resolving technology is the state, where the ferromagnetic samarium substance keeps the electron spin polarization aligning in one direction over a macroscopic range and has little or no magnetization.
This property of a ferromagnetic samarium substance can be basically attained by an appropriate modification of the composition and an appropriate preliminary thermomagnetic process. The reasons for that are described hereinafter.
Samarium usually has five 4f electrons in solids, but sometimes has six ones. In the following descriptions, only the former samarium is dealt with and is just called xe2x80x9csamariumxe2x80x9d hereafter.
For samarium, the spin magnetic moment due to the electrons"" spin polarization and the orbital magnetic moment due to the electrons"" orbital motion are almost the same in size, and couple in an opposite direction as a result of a relativistic effect, called the spin-orbit interaction, to form the total magnetic moment.
Therefore, a ferromagnetic samarium substance is intrinsically characterized by a quite small net magnetization owing to the marginal cancellation between the two parts of the magnetization originating from the spin magnetic moment and the orbital one.
Also, samarium is characterized by the difference in temperature dependence between the spin magnetic moment and the orbital one, which is ascribed to the narrow energy interval of about 1500 K (130 meV) between the ground J-multiplet (the lowest energy state) of the 4f electrons and the first excited one.
In consequence of these two characteristics, the thermal average of the total magnetic moment of samarium in a temperature range below the magnetic transition point strongly depends on the local environment of samarium in solids, and can present unique temperature dependence different from the cases of other magnetic ions. It is known that the local environment of an ion has an influence, to some extent, on the magnitudes of the orbital and/or spin magnetic moment of the ion. The thermal average of the total magnetic moment of samarium, formed by the marginal cancellation between the orbital and spin parts, therefore, is subjected to relatively strong influence as described above, and the temperature dependence can be a unique one reflecting the difference in temperature dependence between the orbital and spin magnetic moments.
Since this unique temperature dependence of the total magnetic moment of samarium becomes easy to understand by comparing with the temperature dependence (thermal magnetization curve) of the magnetization for a ferrimagnetic body, let""s analogize these two cases below.
Various types of the temperature dependence of the magnetization for a ferrimagnetic body are caused by the difference in temperature dependence between the magnetic moments of different kinds of magnetic elements, which couple in an opposite direction, whereas various types of the temperature dependence of the total magnetic moment of samarium in a ferromagnetic samarium substance are caused by the difference in temperature dependence between the spin and orbital magnetic moments of each samarium.
Of such a temperature dependence of the total magnetic moment of samarium, three typical ones are schematically shown in FIG. 1, and it can be seen that these curves resemble the magnetization versus temperature curves for ferrimagnetic bodies.
Since the spin magnetic moment of samarium usually less reduces as the temperature increases in a low temperature range and more steeply varies in the vicinity of the magnetic transition point than the orbital one, each curve shown in FIG. 1 can be interpreted as follows.
Curve (1) having a broad maximum means the temperature dependence of the total magnetic moment of samarium in the case that the spin magnetic moment is larger than the orbital one, and curve (2) gently sloping means the temperature dependence in the case that the orbital magnetic moment is larger than the spin one. Curve (3), where the thermal average of the total magnetic moment becomes zero at a specific temperature below the magnetic transition point, means the temperature dependence in the case that the orbital magnetic moment is larger and smaller than the spin one below and above that temperature, respectively, and the two exactly cancel out just at that temperature, where the thermal average is equal to zero.
As speculated from the aforementioned analogy with a ferrimagnetic body, for a ferromagnetic samarium substance, a perfect compensation between the two parts of the magnetization originating from the spin magnetic moment and the orbital one can be materialized at a specific temperature.
Therefore, by applying an external magnetic field to a ferromagnetic samarium substance at a temperature, where it has a macroscopic magnetization, so as to align the electron spin polarization therein over a macroscopic range, and, subsequently, setting the temperature to be the spin-orbital compensation temperature and reducing the applied field to be nearly zero, the state with the electron spin polarization aligning in one direction over a macroscopic range and no magnetization can be materialized. In that state, the stability of the spin polarization is kept by a strong anisotropy due to the orbital magnetic moment of samarium.
The present invention makes use of such a spin-orbital compensation property, but all ferromagnetic samarium substances do not have this property. It is then required to modify the composition of a ferromagnetic samarium substance to suitably make use of this property. This modification of the composition can be basically attained by an appropriate amount of the atomic substitution or addition to a ferromagnetic samarium substance.
This modification of the composition of a ferromagnetic samarium substance can be performed by either of the following two methods.
One method is to replace a part of constituents in a ferromagnetic samarium substance with non-magnetic elements having no magnetic moment or to add non-magnetic elements to a ferromagnetic samarium substance. Since such a replacement or an addition with non-magnetic elements changes the local environment of samarium, the temperature dependence of the total magnetic moment of samarium can be varied through this change of the environment. It should be noted that this type of the modification of the composition has to be so performed that a remarkable change in magnetic property, especially a change of the alignment of the spin magnetic moment from a ferromagnetic one to an antiferromagnetic one or a considerable fall of the magnetic transition point, does not occur.
Another method is to replace a part of samarium with other magnetic rare-earth element, where the ratio between the spin magnetic moment and the orbital one is different from the case of samarium. Since such a replacement with other rare-earth element changes the ratio between the two partial magnetizations originating from the spin and orbital magnetic moments in the resultant magnetization of a ferromagnetic samarium substance, the net magnetization can be reduced through this change of the spin/orbital ratio.
More particularly, it is generally easy to replace a rare-earth element with the other one, since the ionic radii of rare-earth elements are close to each other. In addition, it is empirically known that, when the spin magnetic moment of rare-earth element aligns ferromagnetically in a base substance, the spin magnetic moment of substitutive rare-earth element basically keeps a ferromagnetic coupling with the neighboring spins.
Therefore, for a ferromagnetic samarium substance, where the orbital part of the magnetic moment of samarium is larger than the spin one, a reduction of the total magnetization and a spin-orbital compensation can be attained by replacing a part of samarium with the heavy rare-earth elements having seven 4f electrons or over, where the spin magnetic moment contributes positively to the total one. Contrarily, for a ferromagnetic samarium substance, where the spin part of the magnetic moment of samarium is larger than the orbital one, they can be attained by replacing a part of samarium with the light rare-earth elements having less than seven 4f electrons, where the spin magnetic moment contributes negatively to the total one.
There is, thus, provided a ferromagnetic samarium substance, which keeps a ferromagnetic alignment of the electron spin therein over a macroscopic range and has no magnetization.
It is understood that the principal driving force for the formation of magnetic domains is the magneto-static energy. For a ferromagnetic material having a spin-orbital compensation property of the invention, on the other hand, there is no positive reason for the formation of magnetic domains because of lack of the magnetization. That is to say, the formation of magnetic-domain structure yields no energy gain and the boundary region concomitant with the formation of magnetic domains (the domain wall) is energetically unfavorable in terms of an anisotropic nature of each magnetic moment and the inter-ionic exchange interaction.
Furthermore, the magnetic moment of the magnetically ordered samarium usually has a strong anisotropy due to the orbital magnetic moment, and the direction is not drastically changed in solids, unless a demagnetizing field and/or an external magnetic field higher than a specific value is applied in that direction. Therefore, once a ferromagnetic samarium substance keeps a ferromagnetic alignment of the electron spin therein over a macroscopic range, the direction of the electron spin polarization must be stably kept at the magnetization-compensation temperature. Also, since a ferromagnetic samarium substance having a spin-orbital compensation property has little or no magnetization, a stray magnetic field from there is practically zero.
Thus, a ferromagnetic samarium substance having a spin-orbital compensation property is available for the spin-resolving technology, as long as it keeps a ferromagnetic alignment of the electron spin therein over a macroscopic range without generating a magnetization. Especially, this is suitably used as a spin-resolving device for charged particles, which comprises a ferromagnetic material and, nonetheless, is free from the formation of magnetic domains and the leakage of a magnetic field.